We propose to evaluate the balance of benefits and risks of vitamin D3 (2,000 IU/day) and marine omega-3 fatty acid (eicosapentaenoic acid [EPA] + docosahexaenoic acid [DHA], 1g/day) supplements in reducing risk of T2D in the setting of an NIH-funded large-scale randomized trial, the VITamin D and OmegA-3 TriaL (VITAL). The VITAL trial is a randomized, double-blind, placebo-controlled trial specifically designed to evaluate the efficacy of vitamin D and marine omega-3 fatty acid supplements in the primary prevention of cancer and cardiovascular disease (CVD) among 20,000 U.S. men and women. We will test the hypothesis that vitamin D or omega-3 supplementation reduces the risk of T2D among initially non-diabetic subjects participating in the trial. We will further assess whether and to what extent vitamin D or omega-3 supplementation improves glucose tolerance and insulin sensitivity/secretion in a subset of the cohort undergoing biomarker evaluation for these endpoints. Hence, we propose a cost-effective strategy to evaluate the efficacy of the study interventions for diabetes prevention as well as to provide detailed characterization of the physiologic mechanisms that may impart this benefit. Case validation of incident T2D will require collection of detailed diagnostic information from medical record review and/or supplementary questionnaires completed by the participants' physicians. To complement case validation, additional data on diabetes diagnoses and hypoglycemic medication will be retrieved by linking the trial participants with the Centers for Medicare and Medicaid Services (CMS) database. In our substudy, 1,000 participants without a report of diabetes at baseline recruited at four Clinical and Translational Science Center (CTSC) sites across the U.S. (Boston, Chicago, San Francisco, and Houston) will undergo a standard 2-hour oral glucose tolerance test (OGTT) at baseline (pre- randomization) and at 2 years of follow-up (matched for season, by month). Serial measurements of insulin and glucose homeostasis will thereby be ascertained. Glycated hemoglobin A1c (HbA1c) will also be measured. In summary, we believe that the research infrastructure of the parent VITAL trial will offer us a unique cost-effective opportunity to answer important and timely questions about potential benefits of vitamin D or omega-3 fatty acid supplementation in the prevention of T2D, as well as to identify potential risks. In this proposed ancillary study, we request funds to perform diabetes case validation in the VITAL cohort and serial glucose tolerance testing at baseline and follow-up in a subsample. In order to complete pre-randomization assessment of glucose tolerance among participants at the CTSC sites, it is critically important that this ancillary study be undertaken parallel to the placebo run-in enrollment period for the parent VITAL trial, which is scheduled to begin in April 2010.